(a) Technical Field
The present disclosure relates to porous silicon dioxide-carbon composites and a method for preparing high-purity β-phase silicon carbide granular powders using the same. More particularly, it relates to a method for preparing high-purity β-phase silicon carbide granular powders in accordance with a first step of preparing gel wherein carbon compounds are uniformly dispersed in silicon dioxide network structures generated by a sol-gel process using a silicon compound and a carbon compound in a liquid state as raw materials, a second step of preparing porous silicon dioxide-carbon composites, in which the carbon compounds are solidified, dried and then thermally treated to have a high surface area, and a third step of conducting both of a direct reaction between carbon and metallic silicon and a carbothermal reduction between carbon and silicon dioxide at the same time through a two-step heat treatment process of the prepared porous silicon dioxide-carbon composites mixed with the added metallic silicon, wherein the average particle size, particle size distribution and purity of the silicon carbide powder can be adjusted through control of a heating rate, a heat treatment temperature and time during the heat treatment process.
(b) Background Art
Silicon carbide (SiC) is a non-oxide-based ceramic material with superior thermomechanical properties such as high-temperature stability, thermal shock property, etc. and corrosion resistance and chemical resistance, and has been used in various industrial applications requiring corrosion resistance as well as high temperature strength. Recently, silicon carbide has been widely applied to the high-technology industries requiring high performance at high temperature under various sever environments such as LED and semiconductor manufacturing process.
Furthermore, comparing with silicon (Si), silicon carbide has a 10 times wider band gap, 3 times higher thermal conductivity and 10 times higher critical field. Accordingly, silicon carbide is used as a wide band-gap semiconductor material along with GaN, ZnO, AlN, etc. Since silicon carbide is chemically stable and strongly resistant to radiation, it is suitable for the manufacturing of semiconductor devices operating under harsh environments. Also, due to superior thermal conductivity, it is used as a substrate for a vertical LED device. Besides, it is the best suited material for high-output, low-loss power semiconductor devices and power semiconductor devices for high-temperature applications. Therefore, it is expected that silicon carbide will replace silicon (Si) in power conversion semiconductor devices with increasing the market of electrical vehicles.
Various methods have been developed to prepare silicon carbide single crystals for power semiconductors. At present, 6-inch silicon carbide wafers manufactured by the physical vapor transport (PVT) method are commercially available. Because the preparation of silicon carbide single crystals with controlled defects is very difficult compared to other ceramic single crystals growth, they are not mass-produced at low fabrication cost on the market. However, when electric vehicles are mass-produced, a silicon carbide powder semiconductor is expected to take a large part in the semiconductor market. For the growth of silicon carbide single crystals by the PVT method, a high-purity granular silicon carbide powder is necessary for starting materials. However, the open market for the high-purity granular silicon carbide powder has not been established yet.
In the methods reported to date, silicon carbide powders have been prepared using various silicon sources and carbon sources in solid, liquid or gas state. The Acheson method is widely used as a representative method for preparing a silicon carbide powder. The Acheson method is advantageous in that silicon carbide powders can be prepared economically in large amount or large scale because the fabrication process is simple and uses low-cost starting materials. However, the prepared silicon carbide by the Acheson process has a purity of 99.99% or lower and extra processes for the fabrication of silicon carbide powders are necessary because the silicon carbide ingot is prepared by the Acheson process. However, since impurities can be included during the fabrication processes for silicon carbide powders, extra purification processes such as acid cleaning are necessary. Accordingly, the silicon carbide powders prepared by the Acheson process have a limitation to be used for preparing silicon carbide single crystals as a raw material due to low purity.
Also, methods for preparing high-purity granular silicon carbide powders using various silicon sources and carbon sources have been developed. As a specific example of a method for preparing high-purity silicon carbide powders using liquid-state silicon sources and liquid-state carbon sources, silicon dioxide-carbon precursors are prepared using liquid-state silicon compounds such as ethyl silicate, a silicon alkoxide or silane and liquid-state carbon compounds such as a phenol resin, a xylene resin, etc. and then high-purity granular silicon carbide having a size ranging from 100 μm to several millimeters or greater are prepared by conducting the carbothermal reduction at high temperatures of 2100° C. or higher under vacuum atmosphere or inert gas atmosphere such as argon (Ar). As a specific example of a method for preparing high-purity silicon carbide powders using solid-state silicon sources and solid-state carbon sources, ultrahigh-purity granular silicon carbide powders containing nitrogen, boron and aluminum at extremely low concentrations is prepared by heating a solid silicon powder and a solid carbon powder at 1200° C. for 12 hours and then heating at 2250° C. for 1-2 hours under vacuum atmosphere maintaining pressure at 10−5 torr or below. The method of preparing high purity silicon carbide powders using solid-state raw materials is advantageous in that silicon carbide single crystals with low defect concentration and high insulating property can be prepared. But, it is not applicable to the large-scale production at low fabrication cost because of the high cost of preparing the silicon carbide powders at very high temperature under high vacuum atmosphere.
Korean Patent Registration No. 10-0338849 (patent document 1) proposes a method of preparing high-purity silicon carbide powders by two step heat treatments. First of all, silicon carbide powders are synthesized by heat treating the thermal hardened mixture consisting of a silicon source selected from a tetraalkoxysilane and a tetraalkoxysilane polymer and a carbon source such as a novolac-type phenol resin at 500-1000° C. under a non-oxidizing atmosphere. And then, post-heat-treating is performed at high temperature of 2000-2100° C. to prepare the high purity silicon carbide powders.
However, the method of the patent document 1 is complex and complicated because a halogen compound has to be added during every step of heat treatment process to achieve high purity of synthesized SiC powders and the post-heat treatment process consists of repeated heat treatments at high temperature of 2000-2100° C. or higher for 2 or more times.
Korean Patent Registration No. 10-1084711 (patent document 2), which has been recently reported, discloses a method of preparing high-purity fine silicon carbide powders prepared by the following steps: performing gelation of a mixture of a silicon source and a carbon source by using an aqueous acid solution, pulverizing the mixture, which is in a gel-like form, adding silicon powders, and then conducting carbothermal reduction at 1250-1600° C.
Korean Patent Registration No. 10-1116755 (patent document 3) proposes a method of preparing β-phase silicon carbide-carbon-silica (β-SiC/C/SiO2) composite powders. According to the method, the silicon carbide precursor powders consisting of silica (SiO2) and carbon are prepared by heat treating hardened gel powders, in which a mixture of a silicon course and a carbon source is subjected to a gelation and then hardened, and then β-phase silicon carbide-carbon-silica (β-SiC/C/SiO2) composite powders are prepared by conducting heat treatment at the temperature of 1300-1600° C.
And, Korean Patent Publication No. 10-2014-0049664 (patent document 4) proposes a method of preparing α-phase silicon carbide granular powders. Fine β-phase silicon carbide powder agglomerates are prepared by a spraying process with an organic solvent using fine β-phase silicon carbide powders synthesized by a carbothermal reduction process. And then, α-phase silicon carbide granular powders are prepared by heat-treating the prepared fine silicon carbide powder agglomerates, at 2000-2200° C. The method for preparing high-purity α-phase silicon carbide granular powders proposed in the patent document 4 has the problems that it requires heat treatment at high temperature of 2000-2200° C. under a vacuum or inert gas atmosphere for a long time and the production cost is high because it involves multiple heat treatment steps at high temperature.
Because it is difficult to prepare granular silicon carbide powders economically with the existing methods described above, process improvement is necessary to enable economical preparation of high-purity granular silicon carbide powders at lower temperatures with high reliability.